The cylinder liner forms the cylindrical space in which the piston reciprocates.

The
reasons for manufacturing the liner separately from the cylinder block (jacket) in which it
is located are as follows;

 The liner can be manufactured using a superior material to the cylinder block.
While the cylinder block is made from a grey cast iron, the liner is manufactured
from a cast iron alloyed with chromium, vanadium and molybdenum. (cast iron
contains graphite, a lubricant. The alloying elements help resist corrosion and
improve the wear resistance at high temperatures.)

 The cylinder liner will wear with use, and therefore may have to be replaced. The
cylinder jacket lasts the life of the engine.

 At working temperature, the liner is a lot hotter than the jacket. The liner will
expand more and is free to expand diametrically and lengthwise. If they were cast
as one piece, then unacceptable thermal stresses would be set up, causing fracture
of the material.

 Less risk of defects. The more complex the casting, the more difficult to produce
a homogenous casting with low residual stresses.

The Liner will get tend to get very hot during engine operation as the heat energy from
the burning fuel is transferred to the cylinder wall. So that the temperature can be kept
within acceptable limits the liner is cooled.

.
 Cylinder liners from older lower powered engines had a
uniform wall thickness and the cooling was achieved by
circulating cooling water through a space formed between
liner and jacket. The cooling water space was sealed from
the scavenge space using 'O' rings and a telltale passage
between the 'O' rings led to the outside of the cylinder block
to show a leakage.

To increase the power of the engine for a given number of cylinders, either the efficiency
of the engine must be increased or more fuel must be burnt per cycle. To burn more fuel,
the volume of the combustion space must be increased, and the mass of air for
combustion must be increased. Because of the resulting higher pressures in the cylinder
from the combustion of this greater mass of fuel, and the larger diameters, the liner must
be made thicker at the top to accommodate the higher hoop stresses, and prevent cracking
of the material.

If the thickness of the material is increased, then it stands to reason that the working
surface of the liner is going to increase in temperature because the cooling water is now
further away. Increased surface temperature means that the material strength is reduced,
and the oil film burnt away, resulting in excessive wear and increased thermal stressing.

The solution is to bring the cooling water closer to the liner wall,
and one method of doing this without compromising the strength
of the liner is to use tangential bore cooling.
 Holes are bored from the underside of the flange formed by the increase in
liner diameter. The holes are bored upwards and at an angle so that they approach the internal
surface of the liner at a tangent. Holes are then bored radially around the top of the liner so that

they join with the tangentially bored holes

On some large bore, long stroke engines it was found that the undercooling further down the liner
was taking place. Why is this a problem? The hydrogen in the fuel combines with the oxygen and
burns to form water. Normally this is in the form of steam, but if it is cooled it will condense on
the liner surface and wash away the lube oil film. Fuels also contain sulphur. This burns in the
oxygen and the products combine with the water to form sulphuric acid. If this condenses on the
liner surface, then corrosion can take place. Once the oil film has been destroyed then wear will
take place at an alarming rate. One solution is to insulate the outside of the liner so that there was
a reduction in the cooling effect. On the latest engines, the liner is only cooled by water at the
very top, relying on the air in the scavenge space to cool the lower part of the liner.
 The photo shows a cylinder liner with the upper and mid insulation
bands known as "Haramaki"
Although Haramaki is a type of Japanese armour, the word also means literally " Stomach or
Body Warmer". i.e an insulator.

Cylinder lubrication: Because the cylinder is separate from the crankcase there is
no splash lubrication as on a trunk piston engine. Oil is supplied through drillings in the
liner. Grooves machined in the liner from the injection points spread the oil
circumferentially around the liner and the piston rings assist in spreading the oil up and
down the length of the liner. The oil is of a high alkalinity which combats the acid attack
from the sulphur in the fuel. The latest engines time the injection of oil using a computer
which has inputs from the crankshaft position, engine load and engine speed. The correct
quantity of oil can be injected by opening valves from a pressurized system, just as the
piston ring pack is passing the injection point.

As mentioned earlier, cylinder liners will wear in service. Correct operation of the engine (not
overloading, maintaining correct operating temperatures) and using the correct grade and quantity
of cylinder oil will all help to extend the life of a cylinder liner. Wear rates vary, but as a general
rule, for a large bore engine a wear rate of 0.05mm/1000 hours is
acceptable. The liner should be replaced as the wear approaches 0.8 - 1%
of liner diameter. The liner is gauged at regular intervals to ascertain the
wear rate.
 Gauging a Liner

It has been known for ships to go for scrap after 20 + years of operation with some of the original
liners in the engine.

As well as corrosive attack, wear is caused by abrasive particles in the cylinder (from bad
filtration/purification of fuel or from particles in the air), and scuffing (also known as
micro seizure or adhesive wear). Scuffing is due to a breakdown in lubrication which
results in localized welding between points on the rings and liner surface with
subsequent tearing of microscopic particles . This is a very severe form of wear.